The present invention relates to an analog voltage signal comparator circuit for comparing an input analog voltage signal with an estimated analog voltage signal, in particular, for use in an ADM (Adaptive Delta Modulation) voice synthesizer.
FIG. 1 shows a conventional comparator circuit for comparing an input analog voltage signal with an estimated analog voltage signal, for use in an ADM voice synthesizer. Analog voltage signal source 11--for example--a microphone, receives a voice signal, and supplies an analog voltage signal Vin corresponding to the voice signal to amplifier 12. An output terminal of amplifier 12 is connected via resistor 13 to node N of an output terminal of D/A converter 14 and a non-inverting input terminal (+) of comparator 15. Analog voltage signal V.sub.N on node N is supplied to the non-inverting input terminal (+) of comparator 15. An analog reference voltage Vref is supplied to an inverting input terminal (-) of comparator 15. Comparator 15 compares analog voltage signal V.sub.N with analog reference voltage Vref and outputs the comparison data to ADM (Adaptive Delta Modulation) circuit 16. ADM circuit 16 performs a predetermined arithmetic operation based on the comparison data to produce an estimated value of input voltage signal Vin, and outputs a digital voltage signal of the estimated value. D/A converters 14 and 17 receive the estimated digital signal and convert it into an analog voltage signal. The analog voltage signal output from D/A converter 14 is supplied to node N, and the analog voltage signal output from D/A converter 17 is supplied to speaker 19 via amplifier 18.
The resistance value of resistor 13 is equalized to an impedance of D/A converter 14, obtained when it is viewed from node N.
The analog voltage on node N is determined by the analog output voltage signal of D/A converter 14 and an output analog voltage signal of amplifier 12.
The voltage level of the initial voltage signal SS supplied to ADM circuit 16 is set, for example, 2.5 V, when the estimated voltage of analog voltage signal Vin is 0 V to 5 V. First, a digital voltage signal corresponding to the initial voltage signal is supplied to D/A converter 14. D/A converter 14 converts the digital voltage signal to an analog voltage signal, and then outputs this signal to node N. The input analog voltage signal Vin from analog signal voltage source 11 is supplied to node N via amplifier 12 and resistor 13. Analog volta V.sub.N, determined by the output of D/A converter 14 and the output of amplifier 12, is supplied to the non-inverting input terminal (+) of comparator 15. Comparator 15 compares voltage V.sub.N with reference voltage Vref supplied to the inverting input terminal (-) thereof. The comparison data is fed to ADM circuit 16, which performs an arithmetic operation to provide the estimated value of the input analog voltage signal Vin. The result (digital signal) of the arithmetic operation is converted into an analog voltage signal by D/A converter 14. The foregoing operation is repeated.
In the case of taking out the estimated analog voltage signal from whole system, the output of ADM circuit 16 is fed to D/A converter 17 and is amplified by amplifier 18. The amplified signal is supplied to speaker 19, which outputs a voice signal corresponding to the estimated analog voltage signal.
D/A converter 17 is employed in order to take out the estimated analog voltage signal from the whole system. The comparator circuit of FIG. 1 is, however, not suitable for an IC, since the D/A converters require a large area on the IC chip.
FIG. 2 shows an improvement of the circuit of FIG. 1. The numerals as used in FIG. 1 indicate the same elements in FIG. 2. Referring to this Figure, a voltage signal Vin output from analog voltage signal source 11 is fed to a non-inverting input terminal (+) of comparator 15 via amplifier 12. An inverting input terminal (-) of comparator 15 is supplied with output voltage signal V.sub.N of D/A converter 14. Comparator 15 compares voltage signal Vin with voltage signal V.sub.n, and supplies the comparison data to ADM circuit 16. ADM circuit 16 feeds a digital voltage signal of an estimated value of the input analog voltage signal to D/A converter 14. Output voltage signal V.sub.N of D/A converter 14 is fed to the inverting input terminal (-) of comparator 15 and also to an input terminal of amplifier 18. Amplifier 18 amplifies voltage signal V.sub.N and feeds it to speaker 19.
As opposed to the structure of FIG. 1, the above-described structure is suitable for an IC, since only one D/A converter is employed, and the area required for the D/A converter can be reduced. However, the characteristics of ADM circuit 16 are restricted by the dynamic range of comparator 15. It is, therefore, necessary to pay attention to the level of the analog voltage signal. Unless the level of the analog voltage signal is specified, the S/N ratio and resolution may be degraded by the restricted characteristics of ADM circuit 16. When a D/A converter which includes a R-2R ladder resistor circuit is used as D/A converter 14, the output thereof varies in a range of the power source voltage. Thus, comparator 15 may be forced to operate in an undesirable range.
As has been described above, in the case of a conventional analog voltage signal comparator circuit, an additional D/A converter is required in order to produce an estimated analog voltage signal. The D/A converter requires a corresponding area on the IC chip. Further, when an estimated analog voltage signal is produced by a single D/A converter, the level of an input analog voltage signal may be restricted by the dynamic range of the comparator, with the result that the S/N ratio and resolution may be degraded.